Microencapsulation process

ABSTRACT

Microcapsules are formed in the absence of coacervation by providing an oil-in-water emulsion containing a polymeric, emulsifying agent having cross-linkable groups or complexing sites and admixing with the emulsion a cross-linking agent or a complexing agent which forms an impermeable coating around the dispersed oil droplets. The emulsifying agent may be non-proteinaceous or the protein, gelatin. Impermeable capsule walls are formed solely by the addition of the cross-linking or complexing agent and extraneous hardening agents are obviated. Moreover, the emulsifying agent may be a preformed, polymeric, cross-linking agent which eliminates the need for any separate cross-linking agent.

This is a continuation of application Ser. No. 174,045 filed Aug. 23,1971 in the name of A. E. Vassiliades, now U.S. Pat. No. 3,886,084,which, in turn, is a continuation-in-part of U.S. Application Ser. No.583,046 filed Sept. 29, 1966, in the name of A. E. Vassiliades nowabandoned.

This invention relates to the microencapsulation of oils. Morespecifically, this invention pertains to processes for encapsulatingminute oil droplets, to microcapsules produced thereby, and to the usethereof in pressure-responsive, transfer-copy systems.

Microcapsules containing both liquid and solid nucleus materials havefound widespread acceptance in a variety of commercial applications. Forexample, one of the most widespread uses has been in the art oftransfer-copy systems wherein minute droplets of a colorless dyeintermediate dispersed or dissolved in an oil are encapsulated andcoated onto a transfer sheet. The dye intermediate is thereaftertransferred to a copy sheet by rupturing said capsules. The underlyingcopy sheet has an adsorbent coating thereon containing a material whichwill react with the dye intermediate causing a visible colored mark atpoints where the microcapsules have been ruptured and the dyetransferred. Other recent applications in which microcapsules have beenused extensively are in adhesives and adhesive tapes, fertilizers,pharmaceuticals, foods and cosmetics. In the majority of theseapplications, microencapsulation involves the "coacervation" phenomenon.

Coacervation is the term applied to the ability of a number of aqueoussolutions of colloids, to separate into two liquid layers, one rich incolloid solute and the other poor in colloid solute. Factors whichinfluence this liquid-liquid phase separation are: (a) the colloidconcentration, (b) the solvent of the system, (c) the temperature, (d)the addition of another polyelectrolyte, and (e) the addition of asimple electrolyte to the solution. This phenomenon is extensivelydescribed in the book Colloid Science edited by H. R. Kruyt, Volume II,Reversible Systems (published in 1949 by the Elsevier PublishingCompany, Inc.), particularly in Chapter VIII entitled"Crystallisation-Coacervation-Flocculation"; Chapter X, entitled"Morphology of Coacervates", by H. G. Bungenberg deJong.

A unique property of coacervation systems is the fact that the solventcomponents of the two phases are the same chemical species. This is amajor distinguishing characteristic of coacervates as compared to twophase systems involving two immiscible liquids. Thus, a colloidal soluteparticle migrating across the interface of a two-phase coacervate systemfinds itself in essentially the same environment on either side of theinterface. From the viewpoint of composition, the difference between thetwo phases is a difference in concentration of solute species.Structurally, the two phases differ in that the colloidal solute of thecolloid-poor phase is randomly oriented and the colloidal solute of thecoacervate or colloid-rich phase shows a great deal of order. In allcases where coacervation has been observed, the solute species aregeometrically anisotropic particles.

Coacervation can be of two general types. The first is called "simple"or "salt" coacervation where liquid phase separation occurs by theaddition of a simple electrolyte to a colloidal solution. The second istermed "complex" coacervation where phase separation occurs by theaddition of a second colloidal species to a first colloidal solution,the particles of the two dispersed colloids being oppositely charged.Generally, materials capable of exhibiting an electric charge insolution (i.e. materials which possess an ionizable group) arecoacervable. Such materials include natural and synthetic macromolecularspecies such as gelatin, acacia, tragacanth, styrene-maleic anhydridecopolymers, methyl vinyl ether-maleic anhydride copolymers,polymethacrylic acid, and the like.

With both simple and complex coacervate systems, a necessaryprecondition for coacervation is the reduction of the charge density ofthe colloidal species. In the case of simple coacervation, thisreduction of the charge density along with partial desolvation of thecolloidal species is similar to that preceding the flocculation orprecipitation of a colloid with the addition of a simple electrolytesince it is known that the addition of more electrolyte to a simplecoacervate leads to a shrinking of the colloid-rich layer and thesubsequent precipitation of the colloidal species. This same reductionof charge density along with partial desolvation of the colloidalspecies which precedes the precipitation of two oppositely chargedcolloids from solution may also be regarded to be the cause for thephase separation in a complex coacervate system. However, while thereduction of the charge density is a necessary precondition forcoacervation, it is oftentimes not sufficient for coacervation.

In other words, the reduction of the charge density on the colloidalparticles must alter or modify the solute-solute interactions to such anextent that the colloidal particles will tend to aggregate and form adistinct, continuous liquid phase rather than a flocculant or a solidphase. This tendency is attributable to both coulombic and long-rangeVan der Waal's interactions of large aggregates in solution. Thus, inboth "simple" and "complex" coacervation, two-solution phase formationbegins with the colloidal species aggregating to form submicroscopicclusters; these clusters coalesce to form microscopic droplets. Furthercoalescense produces macroscopic droplets which tend to separate into acontinuous phase. This phase appears as a top or bottom layer dependingupon the relative density of the two layers.

If, prior to the initiation of coacervation, a water-immisciblematerial, such as an oil, is dispersed as minute droplets in an aqueoussolution or sol or an encapsulating colloidal material, and then, asimple electrolyte, such as sodium sulfate, or another, oppositelycharged colloidal species is added to induce coacervation, theencapsulating colloidal material forms around each oil droplet, thusinvesting each of said droplets in a liquid coating of the coacervatedcolloid. The liquid coatings which surround the oil droplets mustthereafter be hardened to produce solid-walled microcapsules.

For example, in U.S. Pat. No. Re. 24,899 to Green wherein the phenomenonknown as simple coacervation is employed in the formation ofmicrocapsules, a gelable colloid such as the proteinaceous material,pigskin gelatin, is emulsified and then caused to form a liquid coatingaround an oil droplet. The liquid coating is thereafter gelled bycooling in order to form the microcapsular wall. Subsequent to theformation of the gelled wall, it is cross-linked or hardened by the useof formaldehyde as a cross-linking agent for the gelatin. Thus, suchsystems involve: (1) a phase separation step wherein a liquid coating isformed around the droplet; (2) a cooling step wherein the liquid wall isgelled; and (3) a hardening step in which the gelled wall iscross-linked.

Similarly, other microencapsulation processes, such as that described inU.S. Pat. No. 3,137,631 to Soloway, employ a proteinaceous material,i.e., a heat denaturable protein such as egg albumin, which is denaturedto form the microcapsular shell. In order to impart increased stabilityto the capsule wall, the use of known cross-linking agents for proteins,such as formaldehyde and glyoxal are suggested.

Still another technique, preferably involving gelable materials isdescribed in U.S. Pat. No. 3,201,353 to Corbin, which patent disclosesthe employment of a water-soluble zirconium-containing compound toprecipitate a proteinaceous, hydrophilic colloid, e.g., gelatin, inorder to encapsulate a water-immiscible material. As in the previouspatents described, it is indicated that the capsules can be hardened byemploying formaldehyde (column 4, lines 57, et sequa of the patent).

Thus, in processes involving coacervation, a liquid wall is first formedabout an oil droplet, which wall must thereafter be hardened, while inother processes, for example, the Soloway and Corbin processespreviously mentioned, the capsule walls are formed by denaturing a heatdenaturable protein or precipitating a zirconium-containing complex,respectively, and thereafter hardening to provide increased stabilityand presumably impermeability of the capsule shell.

A more recently issued patent relating to microencapsulation, viz, U.S.Pat. No. 3,516,941 to Matson, describes the formation of an impermeableshell of a urea-formaldehyde polymer by the acid-catalyzedpolymerization of a low molecular weight amino aldehyde precondensate.This patent specifically indicates that wetting agents or emulsifiersmust be substantially excluded.

In contrast to the present invention, one of the primary disadvantagesof the coacervation encapsulation techniques is the fact that criticalcontrol over the concentrations of the colloidal material and thecoacervation initiator must be maintained. That is, coacervation willoccur only within a limited range of pH, colloid concentration and/orelectrolyte concentration. For example, in simple coacervation, if adeficiency of the electrolyte is added, two-phase formation will notoccur whereas, if an excess is added, the colloid will precipitate as alumpy mass. With complex coacervation systems using a colloid having anisoelectric point, pH is especially important since the pH must beadjusted and maintained at a point where both colloids have oppositecharges. In addition, when a gelable colloid, such as gelatin, is usedas the encapsulating material, coacervation must take place at atemperature above the gel point of the colloid.

It is therefore, the object of this invention to provide a process forthe microencapsulation of oils which is devoid of the coacervationphenomenon and the disadvantages inherent therewith.

It is another object of this invention to provide oil-containingmicrocapsules without the necessity for a particular electrolyticconcentration or a coacervating agent.

It is yet another object of this invention to provide oil-containingmicrocapsules comprising walls of either non-gelable or gelablecolloids.

It is another object of this invention to provide a pressure-sensitiveand responsive transfer sheet record material comprising a coating ofmicrocapsules applied to one side of a web material, said microcapsulescontaining a colorless dye intermediate dispersed or dissolved in an oiland said microcapsules having been prepared by the process of thisinvention.

These and other objects and features of this invention will becomeapparent from the following description of the invention.

According to the present invention, a process is provided for theformation of microcapsules in the absence of coacervation, whichprocess, in general, includes forming a primary oil-in-water emulsion,which emulsion comprises a water-immiscible oily material dispersed inthe form of microscopic droplets in a colloidal solution of one or moreemulsifying agents, said oily material and said emulsifying agent oragents having about the same hydrophilic-lipophilic balance (HLB), andat least one of the said emulsifying agents possessing groups capable ofreacting with a cross-linking or complexing agent to form an oilimpermeable coating around the dispersed microscopic droplet.

A cross-linking or complexing agent is slowly added to the emulsion withbrisk agitation, and this is continued until the final microcapsules areformed. The emulsion containing the microcapsules may be directly coatedonto a web material, or, alternatively, the microcapsules may beseparated from the emulsion by physical means, such as filtration orcentrifugation, washed to remove any excess oil and, if desired, themicrocapsules may be redispersed in a solution of a binder and coatedonto a web material.

In contradistinction to prior encapsulation processes, the emulsifyingagents of the present invention are "dual functional," i.e., they notonly act as emulsifying agents, but they also form the capsule shell, incontrast to prior processes which employ either: (1) an emulsifyingagent and a separate film forming material; or (2) a film formingmaterial in the substantial absence of an emulsifying agent. Stillfurther, as indicated hereinabove, an impermeable coating is formedabout the oily droplet when emulsification is complete, and thecross-linking or complexing agent has reacted with the emulsifyingagent. The cross-linking and complexing agents of the present inventionare also dual functional. They not only take part in the formation ofthe capsule shell, but they form a hardened capsule wall to an extentthat they eliminate the need for additional hardening agents. Thus,extraneous hardening agents are not required in order to provide animpermeable coating having substantial structural integrity and oilimpermeability, as is the case in prior processes.

According to one aspect of the invention, the present emulsifying agentsare non-proteinaceous, polymeric emulsifying agents possessing groupscapable of reacting with a cross-linking or complexing agent to form animpermeable coating around dispersed microscopic droplets, whenemulsification is complete.

According to a still further aspect of the invention, the emusifier is aproteinaceous material, such as gelatin, which is cross-linked by analdehyde, e.g., formaldehyde, to form an impermeable coating at thecompletion of emulsification and when the cross-linking agent hasreacted with the emulsifying agent.

Thus, in contradistinction to prior encapsulation processes whereingelatin has been employed, there is no need to employ a salt, such as inthe case of simple coacervation, or form a zirconyl complex of thegelatin, or denature the protein by heat or other means prior to formingthe solid-walled microcapsules. Likewise, the subsequent or concurrentemployment of a separate hardening agent is eliminated.

Both the proteinaceous and the non-proteinaceous emulsifying agents ofthe present invention are cross-linked or complexed to form solid-walledmicrocapsules having non-metallic bonds. In addition, the impermeablecoating is formed around the dispersed oil droplets solely by adding thecross-linking or complexing agent of the present invention to theemulsion. For present purposes, the term "solely" as used herein isintended to exclude the addition of extraneous hardening agents or metalcomplexing agents along with or subsequent to the addition of thecomplexing and cross-linking agents of the present invention.

Thus, according to one aspect of the present invention, microcapsulesare formed in the absence of coacervation by the steps of:

(A) forming a primary oil-in-water emulsion, which emulsion comprises awater-immiscible oily material dispersed in the form of microscopicdroplets in a colloidal solution of one or more non-proteinaceous,polymeric emulsifying agents having about the same hydrophil-lyophilbalance as the oily material, and at least one of said emulsifyingagents being selected from the group consisting of an emulsifying agentpossessing cross-linkable groups and an emulsifying agent possessingcomplexing sites;

(B) forming an impermeable coating around said dispersed oil dropletssolely by providing to the emulsion a polymeric or monomericcross-linking or complexing agent such as, for example, polyvinylalcohol, gelatin, gum tragacanth, ethanolamine, ethylene diamine, aborate, methylcellulose, an aldehyde, or an A-stage formaldehydecondensation product.

The cross-linking or complexing agent reacts with the emulsifying agentso as to form an impermeable coating around each dispersed oil dropletand provides microcapsules having structural integrity. The emulsifyingagent and cross-linking or complexing agent are admixed slowly and underconditions of brisk agitation.

According to a second aspect of the invention, microcapsules are formedin the absence of coacervation by a process that is identical to thatpreviously described, with the exception that the emulsifying agent isgelatin and the cross-linking agent is an aldehyde.

According to still another aspect of the present invention, theencapsulating material may also be an emulsifying agent which is selfcross-linking. In such a case, the separate addition of a cross-linkingagent is unnecessary.

The encapsulating material of this invention which encloses themicroscopic oil droplets is an emulsifying agent which is a preformedpolymer and which broadly, has two main characteristics: (1) itpossesses reactive groups capable of reacting with a cross-linking orcomplexing agent to form an impermeable coating about the microscopicoil droplets; and (2) it has an HLB balance similar to that of the oilemployed. As previously indicated, the encapsulating material may alsobe an emulsifying agent which is self cross-linking.

Exemplary of emulsifying agents having the aforesaid characteristicswhich permit their employment in the instant invention are:non-proteinaceous, polymeric, materials such as naturally-occurringcolloids including gums and polysaccharides, such as gum tragacanth, andguar gums. Likewise, synthetic polymeric materials including copolymersof maleic anhydride with an ethylenically unsaturated monomer, such asethylene, styrene, dodecene, methyl vinyl ether, and the like, may besuitably employed. For example, copolymers of methyl vinyl ether andmaleic anhydride are commercially available, for example, from theGeneral Aniline and Film Corporation and are sold under the trademark"Gantrez". These alkali-soluble copolymers have the general structure:##STR1## Other synthetic emulsifying agents include hydroxyl-containingpolymeric materials, such as polyvinyl alcohol, methylcellulose, or thelike.

Proteinaceous emulsifying agents, such as gelatin also possess reactivegroups capable of reacting with a cross-linking agent to form animpermeable coating, and has an HLB balance similar to the oil. Thecross-linking agent is admixed with the gelatin at a temperature abovethe gel point of the gelatin for ease of handling and in order toachieve efficient admixing.

Emulsifying agents which are self cross-linking include, for example,reaction products of an hydroxyl-containing polymeric emulsifying agentsuch as polyvinyl alcohol or methylcellulose, or a copolymer of maleicanhydride and an ethylenically unsaturated monomer, e.g. styrene,ethylene, etc., with a self-condensing thermosetting prepolymer, e.g. aformaldehyde condensation product, such as urea-formaldehyde,melamine-formaldehyde, phenol-formaldehyde, etc. For example, such selfcross-linking emulsifiers may be prepared by adding the self-condensingthermosetting resin prepolymer in an amount of between 10 and about 150percent, preferably between about 40 and 100 percent (based upon theemulsifying agent) to an aqueous solution of the emulsifying agent, e.g.polyvinyl alcohol, (2-20 percent, preferably 5-10 percent), andadjusting the pH to from about 3-6. The resulting mixture is heated forbetween about 2 and about 12 hours at about 60° to about 100° C. and isthen neutralized and cooled in order to inhibit further reaction.

The cross-linking or complexing agents employed with the aforesaidemulsifying agents include, for example, monomeric compounds, such asthe aldehydes, e.g. formaldehyde, glyoxal, glutaraldehyde and otherformaldehyde donors, trioxane, ethanolamine, ethylene diamine, boricacid, the borates, e.g., sodium borate; and macromolecular species, suchas gelatin, gum tragacanth, methylcellulose, and A-stage formaldehydecondensation products.

As previously mentioned, these agents serve the dual function of notonly combining with the emulsifying agent to form the capsule wall, butalso hardening the capsule wall to such an extent that no extraneoushardening agent is required to provide an oil impermeable coating havingstructural integrity.

While some of the cross-linking or complexing agents are suitable foruse with a plurality of emulsifying agents, others are not. Thus,preferred cross-linking or complexing agent-emulsifying agent pairsinclude: (1) gelatin with an aldehyde, such as formaldehyde; (2)polyvinyl alcohol with a borate, e.g., sodium borate; (3) copolymers ofmethyl vinyl ether and maleic anhydride with any of gelatin, gumtragacanth, ethanolamine, ethylene diamine, polyvinyl alcohol; (4) guargum derivatives with any one of a borate, e.g., sodium borate, ormethylcellulose; (5) self cross-linking emulsifiers with themselves; and(6) methylcellulose with an aldehyde, e.g., glyoxal, or an A-stageformaldehyde condensation product, e.g. melamine-formaldehyde.

The term "borate" includes any compound possessing a borate group whichis capable of complexing with the present emulsifying agents, e.g.,polyvinyl alcohol and guar gum to form an impermeable coating. Aspreviously mentioned, walls of the microcapsules of the presentinvention are formed of non-metallic bonds. Boron is considered to be anon-metallic element as defined in The Van Nostrand Chemist'sDictionary, D. Van Nostrand Company, Inc., (1953).

The cross-linking or complexing agent is utilized in amounts sufficientto result in the formation of microcapsules. The relative amounts varywith the particular system, and may be easily determined in each case.However, in contradistinction to prior encapsulation processes, thepolymeric emulsifying agent is dual functional, and serves not only asan emulsifying agent, i.e., to stabilize the surface of the oil dropletand prevent coalescence, but actually provides the capsular shell. Thus,the polymeric emulsifying agent should be provided in relativelysubstantial amounts of, for example, at least about 0.5 part by weightof emulsifier per part of cross-linking or complexing agent. Suitableamounts include, for example, between about one and about 100 parts ofemulsifier, preferably between about one and about 20 parts emulsifierper part by weight of cross-linking or complexing agent.

Emulsification may be conducted at any suitable temperature. Forexample, temperatures in the range of between about 20° C. and about 80°C. may normally be used although temperatures outside this range couldalso be used. If a gelable emulsifying agent is employed, thetemperature must obviously be adjusted so as to prevent gelation duringthe emulsification.

As previously mentioned, a suitable cross-linking agent is an A-stageformaldehyde condensation product, i.e., urea, melamine orphenol-formaldehyde. The term "A-stage" as employed herein is thewater-soluble form of the resin which contains a considerable number ofmethylol groups as defined on page 131 of A Consise Guide to Plastics,by Simonds and Church, Second Edition, Reinhold Publishing Co., N.Y.Thus, the A-stage formaldehyde condensation products of the presentinvention are soluble in water in all proportions in contradistinctionto the thermosetting resins that are employed in the encapsulationprocess of U.S. Pat. No. 3,418,656 to A. E. Vassiliades. Theformaldehyde condensation products employed in that patented process arecapable of separating in solid particle form upon dilution with waterand are thus distinguishable from the resins employed in the process ofthe present invention.

In many prior systems, the formaldehyde condensation product wasemployed as the main film-forming agent, whereas in the present systemit is employed as a cross-linking agent for the emulsifying agent, whichis the main film-forming agent. Accordingly, in prior encapsulationsystems, a very large quantity of the formaldehyde condensation product,e.g., urea-formaldehyde, is employed relative to the emulsying agent, ifan emulsifying agent is, in fact, used. In the present invention, theratio of emulsifying agent to formaldehyde condensation product is atleast 0.5 part by weight emulsifier per part of the formaldehydecondensation product. Preferably, at least about one part to about 4parts of emulsifier per part by weight of the condensation product isutilized. Thus, it is essentially preferred that the admixture that isprovided to form the microcapsules of the present invention consistessentially of a major portion of emulsifying agent and a minor portionof the formaldehyde condensation product on a weight basis.

By "water immiscible oily materials" is meant lipophilic materials whichare preferably liquid, such as oils, which will not mix with water andwhich are inert with regard to the components of the particular system.Low melting fats and waxes may also be used in this invention. However,oils are the preferred nucleus materials since they do not requiretemperature maintenance. In certain embodiments of this invention, thevapor pressure and viscosity of the oily material are to be considered.For example, in the art of making a transfer sheet record material, alow viscosity-low vapor pressure oil is preferred. The viscosity of theoily medium is a determining factor in the speed with which the markingscan be transferred to the copy sheet since low viscosity oils willtransfer more quickly than oils of higher viscosity. The vapor pressureshould be sufficiently low to avoid substantial losses of the oilthrough evaporation during the encapsulation operation. A compromiseshould, therefore, be made in selecting an oil of medium viscosity whichwill have a reasonable rate of transfer onto the copy sheet and ofreasonably low volatility.

In general, the lipophilic nucleus materials can be natural or syntheticoils, fats and waxes or any combination thereof which will meet therequirements of the use for which the microcapsules are intended. Amongthe materials which can be used are: natural oils, such as cottonseedoil, castor oil, soybean oil, petroleum lubricating oils, fish liveroils, drying oils and essential oils; synthetic oils, such as methylsalicylate and halogenated biphenyls; low melting fats, such as lard;and liquid or low melting waxes, such as sperm oil and lanolin (woolwax).

The amount of emulsifying agent relative to the oily nucleus materialemployed will vary over a wide range depending upon the particularsystem under consideration. However, suitable amounts include betweenabout 5 and about 100 parts of emulsying agent per 100 parts by weightoil, preferably between about 10 and about 50 parts of emulsifying agentper 100 parts by weight oil.

Within the scope of the present invention, the herein-disclosedprocesses may be used to encapsulate an oil alone, or alternatively, theoil may serve merely as a vehicle for carrying another active ingredientor material. In this latter utility, the active material may bedissolved, dispersed or suspended in the oily material. The processes ofthis invention can, therefore, be used to encapsulate medicines,poisons, foods, cosmetics, adhesives or any other material which findsutility in microcapsular form.

In the preferred utility ofthis invention, viz., transfer sheet recordmaterial, these processes may be used to encapsulate an oily printingink, such as may be used in smudge-proof typewriter ribbons or carbonpapers. In such a use, it has been found expedient to encapsulate acolorless, water-insoluble dye intermediate dissolved in the oil, thusavoiding the necessity of removing the residual colored matter from theexternal surfaces of the capsules prior to coating as is required in theencapsulation of printing inks. Colorless dye intermediates are whollyconventional in such utilities and are well known in the art. Exemplaryof the colorless dye intermediates which have been contemplated for usein this invention are leuco dyes, such as, crystal violet lactone andderivatives of bis(p-dialkylaminoaryl) methane such as disclosed in U.S.Pat. Nos. 2,981,733 and 2,981,738. These dye intermediates are colorlessin an alkaline or neutral medium and react to form a visible color in anacidic medium. Thus, when a capsule containing such a compound isruptured and the compound is discharged onto an adsorbent, acidicelectron-acceptor material, such as a paper web coated with an organicor an inorganic acid material, a visible color appears on the adsorbentmaterial at the point of contact.

Inhibitors may optionally be dispersed in the oily material along withthe dye intermediates. Such materials are helpful in preventing thelight and heat degradation of the intermediates during the encapsulationprocedure, especially when elevated temperatures are required, such aswhen a fat is encapsulated. Inhibitors are also considered to aid in thestabilization of the colored marking on the copy sheet against theeffects of the atmosphere. A small amount (generally about 1 to 10percent by weight of the dye) of an inhibitor, such as N-phenyl2-naphthylamine, has been used in the practice of this invention.

The leuco dye intermediates which are mentioned above are, in general,oil soluble. Oils which are inert with respect to the dye and in whichthe dye has appreciable solubility, e.g. above 0.5 grams of dye per 100grams of oil, are preferable.

Certain of the emulsifying agents of the type described above give anacidic solution when dissolved in water. Additionally, the complexing oftwo emulsifying agents may result in an acidic pH. When such materialsare utilized to encapsulate an oily material containing a leuco dyeintermediate, a color would ordinarily be produced, since these dyeintermediates react in an acid medium. To prevent such prematurereaction, a basic species or buffer may be incorporated in the emulsionsystem (usually in the water) in order to maintain a basic pH of thesystem, even when the emulsifying agent or agents do not result in anacid solution, as this will prevent an undesired or premature reactionof the dye intermediate by exposure to atmospheric conditions, e.g.carbon dioxide adsorption from the atmosphere.

Suitable buffer systems include base-inorganic salt combinations, suchas sodium hydroxide-sodium borate decahydrate, while a preferredbuffering agent is sodium carbonate, alone. The amount of bufferingagent is comparatively quite small and is only that amount sufficient toprevent a premature acid reaction of the dye intermediate. In general,from 0.05 to 01. gram-equivalents of the material such as sodiumcarbonate, per 3 grams of dye will suffice for such purposes. Such amaterial in the prescribed amounts does not interfere with the colorreaction of the dye intermediates once they have been transferred to acopy sheet containing an electron-acceptor adsorbent material.Ordinarily, a buffer system need not be employed when the encapsulatedmaterial is not acid reactive.

As previously mentioned, the selected emulsifying agent or combinationof emulsifying agents must have a hydrophillipophil balance (HLB)similar to that of the oil used. Based on experimental data, most of thecommon oils and emulsifying agents have ascribed HLB values (seeRemington's Practice of Pharmacy, 11th edition, Mack Publishing Company,1956, at page 191, the disclosure of which is incorporated herein byreference). Thus, by using these figures, the emulsifying agent orcombination of emulsifying agents can be selected to match the HLB valueof the particular oil utilized. If the HLB value for the emulsifyingagent(s) is dissimilar to that of the oil, an unstable oil-in-wateremulsion results and encapsulation is prevented. For example, anemulsifying agent having an HLB value approximately 10 is necessary toform a stable emulsion of light petroleum in water. As the HLB value forthe selected emulsifying agents proceeds downwardly to about 4, thisoil-in-water emulsion tends to become more unstable and will ultimatelyinvert to a water-in-oil emulsion.

The HLB of blends of two or more emulsifying agents can be calculated byproportion. However, in such combinations, certain antagonisms areevidenced within single classes of emulsifiers. For example, when anaqueous, colloidal dispersion of pigskin gelatin (at a lowered pH) andagar is prepared, the gelatin and agar are incompatible. Thisincompatibility can be explained by the phenomenon of coacervation sinceagar is always a negatively charged colloid and gelatin, at a pH belowits iso-electric point (which is about pH 9), is highly positive. Itfollows, therefore, that the gelatin-agar dispersion will be compatiblewhen in an alkaline medium, i.e., when gelatin is above its iso-electricpoint. Similarly, gelatin is compatible (for the purposes of thisinvention) with copolymers of methyl vinyl ether and maleic anhydride,which copolymer forms a negatively charged colloid, when the gelatin isat a pH above its iso-electric point, i.e., a negatively chargedcolloid.

In the case where the HLB balance of the oily material has to be matchedby a combination of two or more emulsifying agents, at least one of theemulsifying agents should be capable of cross-linking or complexing withthe added cross-linking or complexing agent.

The water may be added to the emulsifying agent-oil mixture eitherquickly or slowly with agitation. If the water is added slowly to theoil phase containing the emulsifying agent or agents, a water-in-oilemulsion is formed, which eventually is inverted to an oil-in-wateremulsion with the further addition of water. Such an inversion stepresults in a more stable emulsion with some systems, e.g amethylcellulose emulsifier system.

The ultimate size of the microcapsules is dependent upon the speed ofthe mixing during the emulsification process. Higher mixing speeds willbreak up the oil phase of the emulsion into smaller droplets and therebyproduce smaller capsules. In some instances, such as when awater-insoluble dye intermediate is dissolved in the oily material andthe resulting microcapsules are to be utilized in producing transfersheet record material, the smaller capsules are preferred since they canbe packed more closely to each other. When the capsules are closelypacked, a more uniform marking results (i.e., less discontinuity isobtained) when the microcapsules are ruptured. Microcapsules havingdiameters ranging from 0.1 to several hundred microns can be produced bythe process of this invention. However, capsules having diameters in therange of 0.5 to 5.0 microns are preferred for transfer copy systems.

The temperature of emulsification may be varied over a broad range. Thetemperature must be kept above the gelling point of the emulsifyingagent or agents only if a gelable emulsifying agent is used. Therefore,when a nongelable emulsifying agent is used, e.g. polyvinyl alcohol, thetemperature during emulsification can be varied appreciably withoutaltering the final desired results. Of course, such variation must bekept within reasonable limits, so as not to influence the solubilitiesof the emulsifying agent, encapsulated material, e.g. a dyeintermediate, etc., to an undue extent.

Subsequent to the emulsification process, the cross-linking orcomplexing agent is added to the oil-in-water emulsion, slowly, and withbrisk agitation to form the final microcapsules. Agitation may beachieved by means of a high speed mixer or impeller, by ultrasonic wavesor by other conventional means.

If the emulsifying agent is of the self-complexing variety, thecross-linking or complexing agent comprises the same material as theemulsifying agent, and need not be added in a separate step.

Alternatively, the emulsion containing the microcapsules may be eithercoated directly onto a web material and dried or the microcapsules maybe separated from the emulsion by some physical means such as filtrationor centrifugation; washed to remove any excess oil; redispersed in asolution of a binder; coated onto a web material and dried. Suitablebinders include methyl cellulose, starch, casein, polyvinyl alcohol,synthetic latex, and styrene-butadiene rubber. Alternatively, materialssuch as urea-formaldehyde or melamine-formaldehyde condensates may beemployed.

In one embodiment of the encapsulation process, the oil-in-wateremulsion is prepared by dissolving the emulsifying agent (or agents)with the proper HLB in water and subsequently adding the oily materialto the water solution with agitation until complete emulsification hasoccurred. The emulsion may then be diluted with water to give thedesired viscosity suitable for coating. Care must be taken not toutilize too large an excess of water when a transfer copy system isdesired or the concentration of microcapsules will be reduced and theintensity of the markings produced will be lowered since there will befewer capsules per unit area to be broken. Capsule diameters suitablefor transfer copy systems, i.e., within the 0.5 to 5.0 micron range, arelikewise obtainable by adding cross-linking or complexing agents withagitation as previously described.

The microencapsulated oils of this invention are suitable for use in themanufacture of transfer sheet record material. More specifically,capsules containing a leuco dye intermediate in the oil are to be coatedonto one side of a web material and dried. The coating operation isperformed by conventional means, such as by use of an air knife. Thecapsule coatings are dried at temperatures ranging from about 40° to 75°C. At these temperatures, no appreciable degradation of the capsules,and in particular, the leuco dye intermediate, takes place.

The web material commonly used in transfer sheet record material ispaper and is, therefore, preferable in the practice of this invention.However, the microcapsules produced by the herein disclosed processesare also capable of being coated onto other materials such as plasticand fabric or textile webs. When using a web material having a highdegree of porosity, it is advisable to pre-coat the web with a materialwhich will reduce seepage of the microcapsular coating through the web.Impregnating the web material with polyvinyl alcohol or abutadiene-styrene latex is the conventional practice for producing anessentially impervious substrate.

Transfer sheets made according to the various embodiments of thisinvention have a pleasant appearance and are almost completelysmudge-proof when brought into face-to-face contact with a copy sheetcontaining a coating of an adsorbent electron-acceptor material. Inaddition, they show a marked improvement over the transfer sheetspresently available in commerce. It has been found that coated papercomprising microcapsules which contain a leuco dye intermediatedissolved in the oil and which microcapsules are formed by the processof this invention are extremely stable. For example, exposure of thecoated papers to direct sunlight for five hours, to a temperature of 65°C. for 16 hours, and to a temperature of 60° C. for 17 hours in a 90percent relative humidity environment does not alter either the pleasantappearance or the transfer and color-forming properties of the paper.

The following examples illustrate the best modes contemplated forcarrying out this invention:

EXAMPLE 1

A primary oil-in-water emulsion is formed by adding 50 milliliters ofcottonseed oil containing 2 grams of1-[bis(p-dimethylaminophenyl)methyl]-pyrrolidine (a leuco auramine dyeintermediate) to 10 grams of a purified gelatin having an HLB similar tothat of the oil which is dissolved in 100 grams of water containing 5milliliters 5 N Na₂ CO₃ solution (for the prevention of a prematurereaction of the dye intermediate) at a temperature of about 50° C. overa period of 20 to 30 minutes. Subsequently, 100 milliliters of a 1 Mformaldehyde solution in water are slowly added to the emulsion withbrisk agitation followed by the addition of 50 milliliters of water. Theaddition of the formaldehyde results in the formation of well-definedmicrocapsules.

The microcapsules are then filtered, washed with successive 50milliliter portions of water, methanol and formalin solution, andredispersed in 100 milliliters of water containing 4 grams of a bindingagent comprising methyl cellulose. The solution of methyl cellulosecontaining the microcapsules is coated onto a paper web and dried at50°-60° C.

The following examples illustrate the employment of a non-gelableemulsifying agent in the process of this invention.

EXAMPLE 2

One hundred grams of water, containing 5 grams of polyvinyl alcohol and5 milliliters of 5 N Na₂ CO₃ are emulsified with 35 milliliters ofcottonseed oil (containing 1 gram of1-[bis(p-dimethylaminophenyl)methyl]-pyrrolidine) for a period of 20 to30 minutes. One hundred and fifty milliliters of a 1 M sodium boratedecahydrate solution are slowly added to the emulsion with briskagitation, resulting in the formation of microcapsules which can be seenunder an ordinary microscope. The emulsion containing the microcapsulesis coated onto a paper web and dried at between 50° and 60° C.

EXAMPLE 3

Eight grams of methylcellulose are dispersed in 25 milliliters ofcottonseed oil (containing1-[bis(p-dimethylaminophenyl)methyl]-benzotriazole) and this dispersionis emulsified by the slow addition of 100 milliliters of water. Theaddition of a few milliliters of water (10 to 15) results in awater-in-oil emulsion, which inverts to an oil-in-water emulsion withthe further addition of water. Following emulsification, 3 grams ofJaquar-2Sl (a derivative of guar gum) are slowly added to the emulsionwith brisk agitation, followed by the addition of 100 ml of water(containing 0.1 gram-equivalents of Na₂ CO₃). The addition of the Jaquarresults in the formation of well-defined microcapsules (seen under anordinary microscope) evenly dispersed throughout the emulsion. Theemulsion containing the microcapsules is subsequently coated onto apaper web and dried at about 50° to 60° C.

EXAMPLE 4

Ten grams of gum arabic are dissolved in 100 grams of water and thesolution is emulsified with 25 milliliters of soybean oil (containing 1gram of 1-[bis(p-dimethylaminophenyl)methyl]-benzotriazole).Subsequently, 10 grams of a maleic anhydride-methyl vinyl ethercopolymer (Gantrez-139) are added to the emulsion and emulsification isallowed to proceed for 10 to 15 additional minutes. The subsequentaddition of 10 milliliters of ethylene diamine slowly and with briskagitation results in the formation of well-defined microcapsules. Theemulsion containing the microcapsules is coated onto a paper web anddried at about 50° to 60° C.

EXAMPLE 5

Into a solution of 180 grams of water (containing 25 grams of acopolymer maleic anhydride and methyl vinyl ether (Gantrez) and enoughNa₂ CO₃ bring the pH to 8.5, 50 milliliters of chlorinated biphenylsArochlor No. 1248 containing 2 grams of1[bis(p-dimethylaminophenyl)methyl]-pyrrolidine are added and emulsifiedfor approximately 15 to 20 minutes. To the emulsion, 20 milliliters of10 percent by weight gelatin in water solution are added slowly and withbrisk agitation. The addition of the gelatin solution results inwell-defined microcapsules, evenly dispersed throughout the emulsion.The emulsion is subsequently coated onto a paper web and dried at about50° to 60° C.

EXAMPLE 6

Into 200 grams of water, containing 8 grams of methyl vinyl ether-maleicanhydride copolymer (Gantrez-139) and enough Na₂ CO₃ to bring the pH to8.5, 50 milliliters of castor oil (containing 2 grams of1-[bis(p-dimethylaminophenyl) methyl]-benzotriazole) are added andemulsified. Subsequently, 2 grams of gum tragacanth are added to theemulsion with brisk stirring, resulting in the formation of well-definedmicrocapsules, evenly dispersed throughout the emulsion. The emulsion iscoated onto a paper web and dried at about 50° to 60° C.

EXAMPLE 7

Five hundred grams of a 6 percent by weight aqueous solution ofmethylcellulose (25 centipoises) are charged to a Waring blender. Underconditions of brisk agitation, 100 grams of a solution containing 2.1percent crystal violet lactone and 1.8 percent benzolleucomethylene bluein a 50/50 mixture of chlorinated biphenyl and coconut oil is added andthe agitation continued for about five minutes, or until emulsiondroplets having an average diameter of about four microns are obtained.

Next, the agitation rate is reduced to a point sufficient to maintainefficient mixing and 15 grams of a 40 percent aqueous solution ofglyoxal is added. After mixing for an additional ten minutes, themixture is transferred to a breaker employing mild agitation, where themixture is heated to 60° C. and maintained at this temperature for fivehours to effect cross-linking.

After cooling the resulting capsular suspension, 150 grams of a 5percent aqueous solution of hydroxyethyl cellulose are added. Thedispersion is then coated onto a paper web substrate and dried toprovide a pressure-rupturable transfer sheet.

EXAMPLE 8

The procedure of Example 7 is repeated with the exception that 15 gramsof water-soluble, partially-condensed (i.e., A-stage)malamine-formaldehyde polymer is substituted for the glyoxal and 3.7grams of 20 percent NH₄ Cl is added as a curing (cross-linking)catalyst. An hydroxyethyl cellulose binder is added and the resultingcapsular slurry is coated onto a paper web and dried to provide apressure-rupturable transfer sheet.

EXAMPLE 9

Fifty grams of urea are dissolved in 171 grams of 37 percent aqueousformaldehyde solution in a two-liter, three-necked flask equipped withthermometer, stirrer, reflux condenser and heating mantel. The solutionis adjusted to pH 8 with 10 percent NaOH solution and is then refluxedfor one hour, whereupon 1250 grams of a 6 percent aqueous polyvinylalcohol solution and 3.5 milliliters of glacial acetic acid are added.The reaction is then continued at reflux for six hours and then cooledto room temperature and neutralized to pH 7 with ammonium hydroxide toprovide a self cross-linking reaction mixture.

One hundred grams of the dye solution of Example 7 are emulsified in 784grams of the self cross-linking reaction product in the same manner asdescribed in Example 7. After the emulsification is complete, 12 gramsof 20 percent aqueous NH₄ Cl solution are added to acidify thesuspension and it is then heated for four hours at 60° C. to effectcross-linking. This microcapsular suspension is then formulated into acoating slurry and coated onto a paper substrate.

EXAMPLE 10

Five grams of urea are dissolved in 17.1 grams of a 37 percent aqueousformaldehyde solution, neutralized to pH 8 with 10 percent NaOHsolution, and refluxed for one hour. Then, 294 grams of a 5.1 percentaqueous solution of methylcellulose and 0.8 milliliters of glacialacetic acid are added and the reaction continued at 80° C. for sixhours. A self cross-linking emulsifying agent is thereby produced.

The solution containing the emulsifying agent is placed in a Waringblender and used to emulsify 50 grams of dye solution as described inExample 7. The emulsion is then heated for six hours at 60° C. to give amicrocapsular suspension which is formulated into a coating slurry andcoated onto a paper substrate.

EXAMPLE 11

A mixture of 667 grams of 6 percent aqueous polyvinyl alcohol solution,40 grams of a 65 percent aqueous dispersion of a partially condensed(i.e., B-stage) urea-formaldehyde thermosetting resin, and 7.8 grams of20 percent NH₄ Cl is heated at 60° C. for six hours. After cooling toroom temperature, this self cross-linking reaction product is used toemulsify 100 grams of the dye solution of Example 7. The emulsion isthen heated for four hours at 60° C. to give a microcapsular suspensiononto a paper substrate.

In all of the foregoing Examples, the HLB of the particular oil wasmatched to approximate the emulsifying agents utilized. All percentagesgiven in this application are on a weight basis, unless otherwisespecified.

Although the invention has been described in considerable detail withparticular reference to certain preferred embodiments thereof,variations and modifications can be effected within the spirit and scopeof the invention as described hereinbefore, and as defined in theappended claims.

I claim:
 1. A process for the formation of microcapsules havingnon-proteinaceous walls, in the absence of coacervation, which comprisesthe steps of:(A) forming a primary oil-in-water emulsion, which emulsioncomprises a water-immiscible oily material dispersed in the form ofmicroscopic droplets in a colloidal solution of a non-proteinaceous,polymeric emulsifying agent having about the same hydrophil-lipophilbalance as the oily material, said emulsifying agent being selected fromthe group consisting of an emulsifying agent possessing cross-linkablegroups and an emulsifying agent possessing complexing sites; and (B)forming an impermeable coating around said dispersed oil droplets solelyby adding to the aqueous phase of said emulsion a member different fromsaid emulsifying agent and selected from the group consisting of gumtragacanth and a borate under conditions of brisk agitation, said memberreacting with said non-proteinaceous emulsifying agent so as to form anon-proteinaceous impermeable coating around said dispersed oildroplets.
 2. The process of claim 1 wherein said added member is gumtragacanth.
 3. The process of claim 1 wherein said added member is aborate.
 4. The process of claim 3 wherein said borate is sodium borate.5. The process of claim 4 wherein said emulsifying agent is polyvinylalcohol.
 6. The process of claim 2 wherein said emulsifying agent is acopolymer of methyl vinyl ether and maleic anhydride.